CI/CD: Add GitHub Actions Workflow to validate Swagger documentation

[andrewpollock]

#2485 identified it's possible for the API implementation and Swagger documentation to accidentally become divergent.

It looks like the swagger spec can be generated from the build_swagger.py script. We can make a workflow to run that and check the diff between the generated swagger and the one checked in.

Originally posted by @another-rex in #2486 (comment)